Ask for the proof of posting. That usually results in a lot of squirming by a guilty party. As I've said before to a seller, I'm not saying you haven't posted it, but without any evidence to the contrary I'm keeping all the options open. Remarkably the coin was returned within a week from the lost parcels depot in N. Ireland (instead of the usual 3 months) and forwarded to me. And I refused to let them off the hook by accepting a refund. I said I thought it worth waiting for a coin to appear given it was only costing 10-20% of market value. Especially as the seller had tried it on with someone else who I knew to be decent